Electricity meters are known in the prior art that have a box with a rear face presenting a housing that is arranged to receive a support and that is provided with a locking device for locking the box and for acting in complementary manner on the support to which the box is fastened. By way of example, the support may be a standardized rail of channel section having flanges presenting longitudinal setbacks in the vicinities of their free edges. The locking device generally comprises a catch in the form of a tongue that is mounted to slide against the rear face of the box between a position in which it projects into the housing and a retracted position. The tongue is provided with resilient strips for urging the tongue into the projecting position and it possesses an opposite end projecting laterally from the box in order to enable the tongue to be moved between its two positions.
That solution presents several drawbacks, and the main drawback is that it does not make it simple to access the locking device as a result of electric cables passing close to the end of the tongue that needs to be manipulated. In addition, it does not enable the operator to observe by means of a visual or other marker the position of the catch or whether or not the module is properly held in position, should that be necessary. Finally, manipulating the tongue in order to bring it into the retracted position requires a considerable amount of force and runs the risk of leading to the tongue being damaged.